The structural, vascular, and metabolic heterogeneity of cancer biology confers substantial limitations to the effectiveness of standard cancer treatments, such as radiotherapy or chemotherapy resulting frequently in tumor recurrences. The recurrent disease can be aggressive and often resistant to standard treatments. For example in malignant glioblastoma, 90-95% tumors recur locally and less than 15% of these recurrent tumors show response to standard chemotherapy. De Bonis et al., 115(1):37-43 (2013). Indeed, there is no standard of care for recurrent glioblastoma. The cause of local recurrence is not entirely established and probably reflects the heterogeneity in tumor cells, tumor vasculature and hypoxia—all of which may play a major role, either individually or collectively, in resistance to standard therapies. Given these heterogeneities, information on change in tumor volume alone post-therapy is not predictive of therapy response or recurrence. Surrogate functional markers such as change in tumor vascular density, perfusion and metabolic activity occurring in the tumor due to therapy could provide better prediction of therapy response, aid in stratification of subjects and design of subsequent therapeutic interventions earlier in the disease management. Zhao et al., Journal of Nuclear Medicine; 50(2):239-49 (2009). Therefore importance of non-invasive imaging modalities that provide functional information in predicting therapy response is irrefutable. Tirkes et al., Radiographics; 33(5):1323-41 (2013).
Amongst the emerging vascular targeted therapies, Photodynamic therapy (PDT) is gaining popularity as it causes spatially localized tumor vascular destruction with minimal side effects. In PDT, photosensitizer (PS) preferentially accumulates in malignant tissue as a result of increased tumor vascular permeability and wavelength-specific light-activation of the PS, generates cytotoxic reactive species including oxygen species (ROS). ROS subsequently induces tumor cell and vascular destruction. PDT has temporal and spatial selectivity because (1) PS is accumulated in tumor tissue and its associated vasculature in a time-dependent manner and (2) light illumination can be regionally localized to the area of malignancy. Celli et al., Chem. Rev.; 110(5):2795-838 (2010). Additional tumoral selectivity for the PS can be achieved through liposomal encapsulation or PS conjugation to targeting moieties such as antibodies. Currently, PDT is approved for several applications and is in clinical trials for other malignancies such as Cholangiocarcinoma (NCT01524146), Oropharyngeal Cancer (NCT01718223), Glioma (NCT01682746)16,20,27 and locally advanced pancreatic cancer (NCT01770132).
As with radio- and chemotherapy, heterogeneity in tumor oxygenation can result in variable PDT outcomes. Wilson et al., Lasers Med Sci.; 12(3):182-99 (1997); Woodhams et al., Photochem. Photobiol. Sci.; 6(12):1246 (2007); Henderson B W, Fingar V H., Cancer Research.; 47(12):3110-4 (1987). Besides the inherent erratic nature of tumor vasculature, the heterogeneity of tumor oxygenation is attributed to variations in inter-capillary distance, intra-vascular pO2, blood flow rates, and vascular permeability. Goel et al. Physiological Reviews, 91(3):1071-121 (2011); Diaz-Cano S J. IJMS.; 13(2):1951-2011 (2012); Hockel M, Vaupel P., J Natl Cancer Inst., 93(4):266-76 (2001). Studies such as those by Pogue et al. have shown that as a consequence of this variability, response to PDT treatment is also not homogeneous. Pogue et al., J. Biomed. Opt., 10(4):41206-6 (2005). PDT effectiveness depends, amongst other parameters, on the availability and consumption of oxygen in the target tissue during light delivery and therefore, monitoring of tumor oxygenation before, during, and after PDT could provide an early indication of the long-term treatment outcome.
Direct measurements of tissue oxygenation status during PDT were obtained with pO2 histography. Sitnik et al., British Journal of Cancer, 77(9):1386-94 (1998); Coutier et al., Radiat. Res., 158(3):339-45 (2002). However, micro-electrodes are invasive and only sample volume adjacent to the electrode. Non-invasive optical imaging techniques such as reflectance spectroscopy and diffuse optical tomography used to monitor changes in blood oxygen saturation (StO2), an indirect method to represent tissue oxygenation status, have been promising. Woodhams et al., British Journal of Cancer, 91(4):788-94 (2004); Amelink et al., Journal of Photochemistry & Photobiology, B: Biology, 79(3):243-51 (2005); Yu G., Clinical Cancer Research, 11(9):3543-52 (2005); Wang et al., Cancer Research. October 15; 64(20):7553-61 (2004); Pham et al., Photochemistry and Photobiology. Wiley Online Library; 73(6):669-77 (2001); Kostenich et al., Cancer Letters, 219(2):169-75 (2005); Thompson et al., Appl Opt., 44(19):4023-31 (2005); Sunar et al., Opt Express.; 18(14):14969 (2010). However, these have not provided 3D information on the tumor StO2 heterogeneity that could play a major role in determining PDT outcome.
Blood oxygenation level dependent (BOLD) contrast MRI was also demonstrated for use in PDT, however it was sensitive only to deoxygenated hemoglobin changes in T2* relaxation time and has ˜1 mm spatial resolution. Gross et al., Nature Medicine, 9(10):1327-31 (2003); Kim S-G, Ogawa S., J Cereb Blood Flow Metab., 32(7):1188-206 (2012). Advances in optical and acoustic technologies have led to an increase interest in the use of photoacoustic phenomena for biomedical imaging. This is due to the fact that photoacoustic imaging (PAI) has the sensitivity of optical imaging and yields a 3D atlas of the tumor blood oxygenation status at resolutions equal to ultrasound imaging. Xu M, Wang L V. Photoacoustic imaging in biomedicine. Review of Scientific Instruments; 77(4):041101 (2006). PAI uses a pulsed laser to induce localized thermo-elastic expansion of optical absorber (such as hemoglobin) that generates acoustic waves. The generated photoacoustic waves are detected using an ultrasound transducer. Multiwavelength PAI can provide StO2 and total hemoglobin concentration (HbT) maps without the use of exogenous contrast agents as oxygenated and deoxygenated hemoglobin have different optical absorption properties. Zhang et al., Nat Biotechnol., 24(7):848-51 (2006); Zackrisson et al., Cancer Research, 74(4):979-1004 (2014).